Functional role of skeletal muscle in the innate immune response to sepsis Septic shock is one of the most common yet difficult-to-manage conditions in intensive care. Old or frail patients suffer mortality rates of 30-40%, yet only 4-5% of otherwise healthy adults die. We hypothesize that a key to understanding the effectiveness of defense against septic shock in a given individual may lie in the health and immunoresponsiveness of their skeletal muscles. If true, then novel exercise and nutritional interventions or targeting membrane receptors on muscle may prove effective treatment or prevention strategies. Though the immune responses of skeletal muscle have been known for decades, their specific contribution to host defense has eluded scientists because of difficulty in isolating their net contribution. Our laboratories have developed new transgenic mice that provide the tools to approach this problem effectively for the first time. We provide strong preliminary data to suggest that the influence of skeletal muscle on the response to infection may be strong enough to cause a major shift in our understanding of innate immunity. AIM 1: To determine the contribution of skeletal muscle toll like receptor (TLR) activation on innate immune responsiveness and muscle atrophy in polymicrobial sepsis. The approach to this aim is to study the response to septic shock in conditional transgenic mice that have had an adapter protein for TLRs (Myd88) knocked out in adult skeletal muscle. The importance of this protein in response to infection is made clear by the fact that animals expressing no Myd88 have no cytokine responses to septic shock. We will determine how this mutation in muscle changes the cytokine response profiles over time, the pattern of immune cell trafficking in the circulation and in targeted organs, the extent of tissue damage, the extent of muscle atrophy, acute phase protein expression and the rate of survival. Results will be compared to mutation where Myd88 has been knocked out of inflammatory cells. We will also evaluate whether well-known exercise-induced protection in septic shock operates through changes in immune responsiveness via muscle TLR activation. AIM 2: To determine the contribution of skeletal muscle IL-6 on innate immune responsiveness and development of muscle atrophy in polymicrobial sepsis. IL-6 has a unique role to play in muscle responses to infection. First it is one of the most predominant cytokines secreted by muscle in response to pathogens. Secondly, it is the primary endocrine system responsible for stimulation of acute phase protein secretion in liver and other organs, and it may be an important player in development of atrophy during sepsis. Using a new inducible muscle specific IL-6 knockout mouse, we will determine the contribution of skeletal muscle IL-6 to the overall pattern of host defense, inflammatory cell trafficking, cytokine expression, atrophy, acute phase protein production, organ damage and mortality. In addition, by comparing results with another muscle specific knockout of IL-6 receptor activity, we will determine if the atrophy and acute phase protein responses of muscle in sepsis are due to IL-6 receptor mediated signaling on muscle.